The Preparation and Characterization of Hydrotalcite Thin Films

Owing to the recent focus on reducing CO2 emissions, the preparation and characterization of CO2-selective inorganic membranes and thin films has received considerable attention. Hydrotalcites (HT) are, potentially, good materials for preparing CO2-selective inorganic membranes and films due to their high CO2 adsorption capacity at elevated temperatures and their good thermal stability. The purpose of this study, therefore, was to prepare CO2-selective HT films and to understand their transport characteristics. A vacuum-suction method was used for the synthesis of the HT films, and their properties were characterized using both single and mixed-gas permeation tests, as well as by nitrogen adsorption and by SEM and XRD. The HT films are microporous, as indicated by both the adsorption data, as well as by the fact that the ideal selectivities of pairs of inert gases (e.g., He, N-2, and Ar) through the membrane are higher than the Knudsen values. However, the as-prepared materials contain a substantial fraction of voids and pinholes. Plugging these voids and pinholes using a silicone coating results in CO2-permselective films, which exhibit CO2/N-2 and CO2/He ideal separation factors of 34.4 and 12.4, respectively.